This invention relates generally to thin-film optical coatings and to processes for applying such coatings, and, more particularly, to processes for applying thin-film optical coatings that are hydrophobic and stain-resistant.
Many optical transparencies incorporate coatings made by stacking multiple thin-film layers having precise thicknesses and refractive indices. Generally, when such coatings are derived from inorganic materials, their exposed surfaces are inherently hydrophilic and have a high surface energy. Consequently, foreign contaminants in the form of dirt, oils and fingerprints can adhere strongly to such exposed surfaces.
Such foreign contaminants can severely degrade the optical performance of such coatings by altering the path of impinging light. In addition, the adhesion of such foreign contaminants can be very strong, such that they are removable only by applying various chemical cleaners or by physical wiping. However, such cleaning processes can permanently damage the coatings and/or the underlying substrates.
One prior technique for reducing the surface energy of such inorganic coatings is to thermally treat the coated substrates at temperatures in the range of 400 to 800xc2x0 C., so as to dehydroxylate the surfaces"" xe2x80x94OH groups. This technique generally is inadequate to treat surfaces already exposed to the ambient environment, and it cannot be used to treat coatings applied to substrates formed of plastic, because plastic substrates typically have deformation temperatures lower than the treatment temperature.
Another prior technique for reducing the surface energy of such inorganic coatings includes modifying the surface using a chemical vapor deposition process. Unfortunately, this technique requires the use of relatively expensive vacuum equipment and it requires relatively long time durations for the reaction and for evacuation of chemicals from the chamber.
Yet another prior technique for reducing the surface energy of such inorganic coatings is to deposit onto the optical coating a hydrophobic overcoat containing saturated hydrocarbon, fluorocarbon or organosilane. Organosilane-based solutions ordinarily incorporate non-polar or less polar solvents, such as octane, heptane, toluene, tetrahydrofuran and trichloroethane, which are either highly flammable or toxic. The use of such solvents, therefore, is undesirable. Known techniques for grafting organosilane coatings to underlying coated substrates also require relatively long soaking times under dry atmosphere (dry nitrogen or dry air), followed by rinsing or wiping with solvents and baking at elevated temperatures. Applying the coatings in a higher humidity environment can deteriorate the coating solution and/or cause the coating to be non-uniform or translucent.
In addition, organosilane coatings typically have optical indices very similar to that of glass, such that when such coatings are applied to glass substrates, the coatings might not be readily visible. However, any slight variation in the coating""s thickness can adversely affect the optical performance of many optical substrates with multi-layer thin film coatings and can render surface contamination and defects highly visible.
It should, therefore, be appreciated that there is a need for a process for applying a low-cost, efficient, thin-film optical coating to a transparent substrate, wherein the coating is hydrophobic, oil-repellant, and stain-resistant, and wherein the coating does not adversely affect desirable optical properties of any underlying coating and/or the substrate. The hydrophobic coating also should be chemically stable, i.e., non-reactive with various solvents and detergents, and it should be mechanically and environmentally stable, i.e., resistant to variations in temperature, humidity and ultraviolet light. In addition, the hydrophobic coating should be uniform, with no pin-hole or spot defects, and its use should not be limited to any particular substrate size. Further, the coating should be applied by a simple dip-coating procedure without the need for toxic or highly flammable solvents, and it should be environmentally friendly and should not require the need for wiping or rinsing during the application process. The present invention satisfies these needs and provides further related benefits.
The present invention is embodied in a transparent substrate carrying a thin-film optical coating, and in a process for applying it, wherein the coating is hydrophobic, oil-repellant, and stain-resistant, and wherein the coating does not adversely affect desirable optical properties of any underlying coating and/or the substrate. The hydrophobic coating is chemically stable, i.e., non-reactive with various solvents and detergents, and it is mechanically and environmentally stable, i.e., resistant to variations in temperature, humidity and ultraviolet light. In addition, the hydrophobic coating is uniform, with no pin-hole or spot defects, and its use is not limited to any particular substrate size. Further, the coating is applied without the need for toxic or highly flammable solvents, and it is environmentally friendly and does not require the need for wiping or rinsing during the application process.
More particularly, the hydrophobic, oil-repellant, and stain-resistant thin-film coating is prepared by dip-coating a substrate in a treatment solution incorporating an organosilane in a solvent that includes water, an alcohol such as ethanol, propanol or butanol, ethylene glycol or glycerol, and an acid catalyst. The organosilane preferably has a concentration in the range of 0.05 to 50 mmole per 1000 ml of solution, and the solvent includes 0.1 to 90% by weight water, and 0.5 to 20% by weight ethylene glycol and/or glycerol, with the remainder being an alcohol, preferably ethanol or propanol, alone or mixed with butanol. The treatment solution preferably is prepared in a two-step procedure, in which the organosilane is first reacted in concentrated form and then diluted.
The organosilanes incorporated into the treatment solution have a saturated or fluorinated hydrocarbon chain, which will directly attach to silicon atoms in the underlying substrate by C-Si bonds. The organosilanes also contain 1 to 3 hydrolyzable groups, which directly bond to silicon atoms by Cxe2x80x94Oxe2x80x94Si bonds. The organosilanes can be described by the following general formula:
(RO)3xe2x88x92nxe2x80x94Sixe2x80x94{[Cp+qH2pF2q]xe2x80x94(CH3 or xe2x80x94CF3)}1+n
where:
n is 0-3,
p+qxe2x89xa72 and preferably  greater than 8, and
R is an alkyl group, preferably CH3xe2x80x94 or CH3CH2xe2x80x94.
The acid catalyst incorporated into the treatment solution is a mineral or organic acid, preferably CH3CO2H, HCl or HNO3.
The dip-coating procedure requires the substrates to be immersed in the treatment solution for just one to 60 seconds and to be withdrawn at a rate higher than about 0.001 cm/sec. Any size substrates can be accommodated, and multiple substrates can be dip-coated at a time, with the restriction that the substrates be spaced at least 1 cm apart. The relative humidity during the dip-coating procedure should be in the range of 15 to 95%, and preferably in the range of 40 to 80%. The temperature should be in the range of 10 to 40xc2x0 C. The dip-coated substrates are flash-dried at room temperature for at least one minute, and preferably 20 minutes, and then baked at a temperature in the range of 50 to 250xc2x0 C. for at least one minute.
The surfaces of the substrates to be coated in accordance with the invention should contain metallic or inorganic components, including by way of example, SiO2, Al2O3, ZnO2, TiO2, ITO, In2O3, Sb2, O3, MgF2, and SnO2. These surfaces can be formed by bulk material, by coatings of such components on an underlying substrate, or by organic/inorganic composites containing such inorganic components.
Other features and advantages of the present invention should become apparent from the following description of the preferred embodiments and processes, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.